This invention relates to electronic ballasts for operating discharge lamps such as fluorescent lamps, and in particular to such ballasts used in applications where a variation of lamp operating parameters is undesirable, whether due to ambient temperature, line voltage or other variations.
Most magnetically coupled self-oscillating inverters are manufactured in large quantities for sale in a highly competitive market. Half-bridge inverters generally have a lower cost because of a reduced parts count. Such inverters may be classified into two groups: those using a current transformer having a saturable core, generally together with power BJT's (bipolar junction transistors); and those using a current transformer having a linear core, generally together with MOSFETs (metal oxide semiconductor field effect transistors). As those of ordinary skill will recognize, in this context a linear core is one in which operation is over a region having a curved B-H characteristic, rather than a sharp B-H characteristic; that is, at all times the flux level is such that a significant increase in magnetizing current will be accompanied by a significant increase in flux level.
In designing a lamp ballast with a saturable core, the storage time of the power BJT occupies a large portion of a switching period, and is a complicated function of the forward base current, the reverse base current, the current gain in the saturation mode, the collector current and the minority carrier life time in the base. This large number of variables and affecting factors causes the circuit to be sensitive to its operating environment, and the circuit operating point changes with load and input power line voltage variations, changes in ambient temperature, and the like.
A mathematical analysis of fluorescent lamp converters of the half-bridge MOSFET type is found in an article by L. R. Nerone, "A Mathematical Model of the Class D Converter for Compact Fluorescent Ballasts," IEEE Transactions on Power Electronics, vol. 10, no. 6, Nov. 1995 at pp 708-715. FIG. 3 of this article shows a schematic diagram of such a converter having a current transformer in series with a resonant load circuit. The current transformer has a primary or load current winding T.sub.1c which senses the converter output current and provides control signals directly from output windings T.sub.1a and T.sub.1b on the current transformer to the gates of the MOSFETs. The fluorescent lamp is connected in parallel with a tuning capacitor C, and this combination is in series with a resonance inductor L. This circuit has the disadvantage that it is quite sensitive to the DC voltage supplied to the half-bridge circuit. Footnote 1 on page 710 points out that a variation of this circuit, in which the capacitor current itself is sensed, can sometimes be used to reduce sensitivity to power line variations. Further, as pointed out at page 712, the need for the converter to operate at close to the resonant frequency of the series RLC circuit, in order to develop a high voltage to ignite the lamp, and then operate with correct phase in the gate driving circuit although the load current is lagging, impose strict requirements on the circuit. The situation can be additionally complicated if the current due to gate capacitance becomes significant.